Chapter 16 Amino Acids, Proteins, and Enzymes

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Chapter 16 Amino Acids, Proteins, and Enzymes

• 16.1 Functions of Proteins
• 16.2 Amino Acids
• 16.3 Amino Acids as Acids and Bases

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Functions of Proteins

• Proteins perform many different functions in the
  body.

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Amino Acids

• Amino acids
• are the building blocks of proteins.
• contain a carboxylic acid group and an amino
  group on the alpha (?) carbon.
• are ionized in solution.
• each contain a different side group (R).
• R side chain R
• H2NC COOH H3NC COO-
• H H ionized form

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Examples of Amino Acids

• H
• H3NCCOO-
• H glycine
• CH3
• H3NCCOO-
• H alanine

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Types of Amino Acids

• Amino acids are classified as
• nonpolar (hydrophobic) with hydrocarbon side
  chains.
• polar (hydrophilic) with polar or ionic side
  chains.
• acidic (hydrophilic) with acidic side chains.
• basic (hydrophilic) with NH2 side chains.

Nonpolar Polar
Acidic
Basic
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Nonpolar Amino Acids

• An amino acid is nonpolar when the R group is H,
  alkyl, or aromatic.

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Polar Amino Acids

• An amino acid is polar when the R group is an
  alcohol, thiol, or amide.

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Acidic and Basic Amino Acids

• An amino acid is
• acidic when the R group is a carboxylic acid.
• basic when the R group is an amine.

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Learning Check

• Identify each as (1) polar or (2) nonpolar.
• A. H3NCH2COO- (Glycine)
• CH3
• CHOH
• B. H3NCHCOO - (Threonine)

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Solution

• Identify each as (1) polar or (2) nonpolar.
• A. H3NCH2COO- (Glycine) (2) nonpolar
• CH3
• CHOH
• B. H3NCHCOO - (Threonine) (1) polar

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Fischer Projections of Amino Acids

• Amino acids
• are chiral except glycine.
• have Fischer projections that are stereoisomers.
• that are L are the only amino acids used in
  proteins.
• L-Alanine D-Alanine
  L-Cysteine D-Cysteine

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Zwitterions

• A zwitterion
• has charged -NH3 and COO groups.
• forms when both the NH2 and the COOH groups in
  an amino acid ionize in water.
• has equal and charges at the isoelectric
  point (pI).
• O
  O
• NH2CH2COH H3NCH2CO
• glycine zwitterion
  of glycine

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Amino Acids as Acids

• In solutions more basic than the pI,
• the NH3 in the amino acid donates a proton.
• OH
• H3NCH2COO H2NCH2COO
• zwitterion Negative ion
• at pI pH gt pI
• Charge 0 Charge 1-

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Amino Acids as Bases

• In solution more acidic than the pI,
• the COO- in the amino acid accepts a proton.
• H
  
• H3NCH2COO H3NCH2COOH
• zwitterion Positive ion
• at pI pHlt pI
• Charge 0 Charge 1

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pH and ionization

• H OH
• H3NCH2COOH H3NCH2COO H2NCH2COO
• positive ion zwitterion
  negative ion
• low pH pI high pH

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Separation of Amino Acids

• When an electric current is used to separate a
  mixture of
• amino acids
• the positively charged amino acids move towards
  the negative electrode.
• the negatively charged amino acids move toward
  the positive electrode.
• an amino acid at its pI does not migrate.
• the amino acids are identified as separate bands
  on the filter paper or thin layer plate.

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Separation of Amino Acids

• With an electric current, a mixture of lysine,
  aspartate,
• and valine are separated.

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Learning Check

• CH3 CH3
• H3NCHCOOH H2NCHCOO
• (1) (2)
• Which structure represents
• A. Alanine at a pH above its pI?
• B. Alanine at a pH below its pI?

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Solution

• CH3 CH3
• H3NCHCOOH H2NCHCOO
• (1) (2)
• Which structure represents
• A. Alanine at a pH above its pI? (2)
• B. Alanine at a pH below its pI? (1)